Melted copper and copper-containing alloys, such as bronze, are very reactive to oxygen and become highly oxidized on any copper-containing melt surface exposed to an oxygen-containing atmosphere in contact with the melting metal. In an attempt to minimize oxidation on a copper-containing melt surface, therefore, the melt upper surface has been protected from oxidation by covering the copper melt surface with a reducing melt cover, such as a protective layer of dry charcoal, powdered graphite, flake graphite or the like. The protective layer minimizes exposure of the melted copper to oxygen in the atmosphere within a furnace or other vessel used to melt the copper-containing metal, thereby minimizing copper oxidation. The protective layer is applied to an upper surface of the melting/melted copper-containing metal in a thickness of about one millimeter to about three or four inches, usually about one to two inches.
Brugger U.S. Pat. No. 5,193,604, for example, discloses the use of borax as a protective layer over a melting copper-containing metal, and discloses that the borax can include finely divided mixtures of metals that have an affinity for oxygen, such as Mg, Li, Ce and/or can include powders of graphite and/or fire clay and/or charcoal. As disclosed in the Brugger '604 patent, the borax protective layer causes the formation of copper borides that detrimentally and adversely affect the structure of copper alloys unless the mold is cooled quickly, e.g., with water, as in the centrifugal casting method disclosed, to minimize forming copper borides.
Iron and sulfur are additional elements that one must avoid adding to the melt during the melting of copper and copper alloys to avoid the formation of copper/iron and copper/sulfur reaction products. Also, since copper bonds to silica, silica sand cannot be used as the protective melt-covering layer. The selection of a protective layer for protecting copper and copper alloys from oxidation, therefore, has been a difficult task and, therefore, the art has for the most part, used powdered or flake graphite or charcoal to form an oxidation-resistant protective layer over melting and melted copper and copper-containing metals.
The recommended melting procedure for high-conductivity copper includes the step of deliberately inducing a high oxygen content into the melt to limit the pickup of hydrogen and to oxidize impurities that are deleterious to conductivity. The copper is melted using a protective, deoxidizing cover layer, such as flake graphite. After the molten copper has reached 2300.degree. F. (1200.degree. C.), the furnace is turned off and the graphite covering layer is skimmed off, for example, using a graphite rod.
Equipment used to melt copper and copper alloys, such as brass and bronze, are either fuel-fired or electric. The pouring temperatures of common copper-containing alloys are shown below:
______________________________________ POURING TEMPERATURES OF COPPER ALLOYS Alloy Pouring Temperature Composition Name No. Deg C Deg F ______________________________________ 88 Cu-6 Sn-1.5 Pb- Leaded tin 2A 1,075 to 1,250 1,950 to 4.5 Zn bronze (Navy 2,300 M bronze) 80 Cu-10 Sn-10 Pb High-leaded 3A 1,000 to 1,225 1,850 to tin bronze 2,250 85 Cu-15 Sn-5 Pb- Leaded red 4A 1,075 to 1,300 1,950 to 5 Zn brass 2,350 76 Cu-2.5 Sn- Leaded 5B 1,075 to 1,250 1,950 to 6.5 Pb-15 Zn semi-red 2,300 brass 57.5 Cu-39.25 Zn- Manganese 8A 950 to 1,100 1,750 to 1.25 Fe-1.25 Al- bronze 2,000 .25 Mn (65,000 psi) 64 Cu-26 Zn-3 Fe- Manganese 8C 975 to 1,150 1,800 to 5 Al-4 Mn bronze 2,100 (110,000 psi) 88 Cu-3 Fe-9 Al Aluminum 9A 1,100 to 1,200 2,000 to bronze 2,200 64 Cu-4 SN-4 Pb- Nickel silver 11A 1,225 to 1,425 2,250 to 8 Zn-20 Ni (20% Ni) 2,600 81 Cu-4 Si-15 Zn Silicon brass 13B 975 to 1,150 1,800 to 2,100 96.5 Cu-2.5 Be- Beryllium . . . 1,000 to 1,225 1,850 to 1.1 Ni bronze 2,250 ______________________________________
It has been found that carbon sands that have been used together with a binder to form foundry sand molds in the foundry industry, such as described in this assignee's U.S. Pat. Nos. 5,215,143 and 5,094,289, both patents hereby incorporated by reference, can be used (without a binder) to form a protective layer over copper and copper alloy melts without the formation of copper/sulfur reaction products. The carbon sands have a number of unique advantages over other materials used to date as protective coverings for copper-containing melts, such as ease of separation of the protective carbon sand layer from the melted copper-containing metal; the capability of reusing the carbon sand; the capability of remelting any copper or copper alloy removed with the carbon sand that is skimmed from the melt surface after metal melting is complete; the carbon sands are granular, thereby avoiding dust, yet provide excellent protection against copper oxidation when used at a thickness of about one millimeter up to about three or four inches. Trial tests indicate that the carbon sands transfer little to no sulfur or hydrocarbons into the copper melt, below detection capacity of the sulfur detecting apparatus, and certainly within acceptable limits, at temperatures as high as 2500-3000.degree. C., and probably at higher temperatures, e.g., 4000.degree. C.